Diffusion Data for Clinopyroxenes from Homogenization and Self-Diffusion Experiment

John B. Brady, Smith College
Robert H. McCallister, Purdue University

Abstract

Kinetic experiments involving the homogenization of fine-scale, coherent (001) pigeonite lamellae in a sub-calcic diopside have been used to constrain Ca-Mg interdiffusion coefficients for clinopyroxenes. At 1150°-1250°C and 25 kbar, "average" Ca-Mg effective binary interdiffusion coefficients are described by

Ď = (3.89 x l0^-7) exp(-360.87 kJ/RT) (m²/sec)

(Ď : (3.89 x l0^-3) exp(-86.25 kcal/RT) (cm²/sec))

with an uncertainty of a factor of 2. Because of the large thermodynamic effect on diffusion near a solvus, actual interdiffusion coefficients should vary by an order of magnitude or more with composition for the conditions of these experiments.Attempts to obtain Ca and Fe self-diffusion coefficients in natural diopsides at 1 atm using ⁴⁵Ca and ⁵⁷Fe tracers and thin-fiIm, autoradiographic techniques were unsuccessful.These negative results place upper bounds on the Ca and Fe self-diffision coefficients in diopside that are consistent with the results of the homogenization experiments.